Transformer case grounding pad



A ril 22, 1958 w. A. OKENFUSS 2,331,913

TRANSFORMER CASE GROUNDING PAD Filed April 30, 1951 United States Pa ent2,831,913 7 TRANSFORMER CASE GROUNDING PAD Wilfred A. Okenfuss,University City, Mo., assignor m Wagner Electric Corporation, St. Louis,Mo., a corporation of Delaware Application April 30, 1951, SerialNo."223,762

1 Claim. (01. 174-51 This invention relates to transformers and is moreparticularly directed to an improved ground connection therefor. I

The use of transformers in modern power and distribution circuitsrequires that the casing of the transformer besolidlygrounded as aprotection against failures in the power system which would otherwisecause dangerous voltages to be built up between the transformer case andground. Unless a low resistance ground connection can be made to thetransformer case, an arcing condition could be produced which, by reasonof the inherent inductance and capacitance of thecore and coilconstruction, could cause voltages. to be produced across'parts of thewindings far greater than the voltage for which it is designed, thuscausing failure in the insulation of the transformer. To overcome thepossibility of an arcing ground on a transformer, it is necessary toprovide a low resistance connection to the casing of the transformerwhich will maintain its characteristics without attention over manyyears of service.

One of the objects of the invention is to provide a grounding pad which,when applied to a transformer casing will eliminate previous objections,such as cor- 2,831,913 Patented Apr. 22, 1958 ICC , accidental damage inhandling the transformer, as well as providing for as short a groundingcable 6 as feasible.

Since the grounding cabled and its associated lug 7 are usually made ofcopper in order to secure high conductivity and long life, it isnecessary to provide a contact surface on the grounding pad 4 which willproduce a low resistance junction at all times after connection has beenmade therebetween. It is a wellknown fact that when, any ferrousmaterial is exposed to the atmosphere, it will rust. The rust so formedconsists principally of iron oxide which is a poor conductor ofelectricity and, therefore, it is necessary to face the ground pad whichwill provide the requisite conductivity and, at the same time, a strongassembly that can be readily.

, assembled on the transformer casing.

Referring to Figs. 3- and 4, the grounding pad 4 consists of a faceplate 9,'preferably made of copper, and

bondedto the steel block 10 in'such a manner that there is no chance ofatmosphere reaching the face ofthe block 10 and causing oxidationthereof. The edgesand opposite face of block 10 are painted withrustresisting paint after assembly on the transformer casing, unless thegrounding pad-4 is assembled on the transformer casing rosion in thecontacts, loosening of joints, insufiicient Widely different physicalproperties into a bonded struc- I configuration of grounding pads fortransformers.

In the drawings: 7 Fig. 1 is a transformer casing with the grounding padattached thereto, a

Figure 2 is a side view of the casing of Fig. 1, Fig. 3 is a detaildrawing showing a preferred method I of attaching the grounding pad tothe transformer casing,

Fig. 4 is a detail drawing showing a modification of the method ofattaching the grounding pad to the transformer casing,

Fig. 5 is a perspective view of the fabricated transformer groundingpads prior to separation into individual units; and a Fig. 6 is a detailview of the assembly of the necessary parts prior to bonding.

Referring to Figs. 1' and 2, the transformer casing 1 has a bottom plate2 and a base 3. Welded to the edge of the bottom plate 2 is a groundingpad 4 which serves as a means for making connection between an earthground 5 through the grounding cable 6 provided with a lug 7 securedthereto which, in turn, is removably secured to the grounding pad 4 bycap screws 8. 'Ihe grounding pad may be attached to the transformercasing, as shown in detail in Fig. 3, or may be alternately weldeddirectly ture that can be readily machined to form the desired 7 asshown in Fig. 4 with the opposite face of the block 10 in surfaceabutment with the casing. The lug 7 of the grounding cable 6 is securedto the face of the grounding pad by cap screws 8 (Fig. I) screwed intothe tapped holes 11 projecting through both the copper face 9 and thesteel block 10 of the grounding pad 4.

The grounding pad 4 is fabricated by pre-cutting blocks of steel to therequired dimensions and the copper 12 to such a dimension that severalblocks 10 may be placed thereon, as shown in Fig. 5. Between each steelblock and the copper slab 12 there is placed lengths of low meltingalloy or other suitable bonding material, such as a silver bearing alloysolder known as easy fio, shown at 13 in Fig. 6. The assembly is thenheated in a controlled atmosphere furnace with the copper slab on thebottom, to a temperature suchthat the alloy fuses and, with the aid offlux previously applied, wets the adjacent surfaces of the copper andsteel. The weight of the individual steel blocks and capillaryattraction causes the molten alloy to flow over the entire adjacentsurfaces of the steel blocks and copper slab. The assembly is thenslowly cooled to a point below freezing of the alloy and the assemblyremoved from thefurnace. After the bonded steel blocks and copper slabhave cooled to room temperature, the copper slab is cut by a saw, orother convenient means which will not distort the exposed surface of thecopper, to the lateral dimensions of the steel block. The individualpads 4 then have holes 11 (Fig. 3) drilled and tapped therein, thecopper face being sanded to remove any irregularities and oxides whichmight have formed during the fabricating process. 7 I

The assembly of the ground pad on the transformer casing is accomplishedby welding it to the bottom plate 2 of the casing, as at 14 (Fig. 3), orwelding it to the side wall of the casing 1, as at 15 (Fig. 4). Theadvantage of producingground pads for transformer casings in the mannerdescribed is that bypre-machining the steel block 10 to dimensions andbonding it to the copper slab, the diificulty in economically machininga combination of'copper and steel having such widely varying machiningcharacteristics as to require entirely different tools, is removed. Thismethod requires that only the copper be out after the bonding process.Due to the to drill and tap the holes in the assembly.

Another advantage of the grounding pad above described, is that thefabricated pad may be welded on the transformer casing, whereas should asolid copper pad be used, it would be necessary to braze the joint whichis both costly and difficult. When the grounding pad 4 is utilized as inFig. 3, it is an advantage to have the steel block backing up the copperface 9 to supply the necessary strength for preventing damage byaccidentally hitting it in any manner.

While the grounding pad has been described in detail in connection withits use on transformer casings, it is evident that no modificationwhatever would be required for its use on any steel structure to which aground must be attached.

What I claim is:

In a device of the type described including an electrically conductivemetallic casing for a transformer and a non-corrosive metallicelectrical grounding connector for the casing, the improvement whichcomprises a grounding pad for providing a low resistance non-corrosiveconnection between the casing and the grounding connector, saidgrounding pad comprising a relatively thick ferrous metal block withfaces having an area substantially larger than the cross-sectional areaof the grounding connector, one of the faces being secured in contiguousabutting relationship with the casing to provide an intimate electricalcontact therebetween, a relatively thin non-ferrous non-corrosive metalfacing having an area similar to the area of the faces of said block,said facing being bonded to the other face of said block in imperviousabutting relationship, and an internally I 2,831,913 H r f threadedaperture extending through said facing andihto the block for securingthe grounding connector to said grounding pad, the grounding connectorhaving a lug with a substantially flat surface in abutment with saidfacing over an area larger than the cross-sectional area of thegrounding connector whereby a low resistance non-corrosive electricalconnection having an area larger than the cross-sectional area of thegrounding connector is provided between the grounding connector and thecasing.

References Cited in the tile of this patent- UNITED STATES PATENTS970,693 Donald Sept. 20, 1910 1,792,552 Siegmund Feb. 17, 1931 1,813,448Horton July 7, 1931 1,837,238 Siegmund Dec. 22, 1931 1,907,932 Pugh May9, 1933 2,137,617 Imes et a1. Nov. 22, 1938 2,181,083 Payette Nov.21,1939 2,264,271 Barnsteiner Dec. 2, 1941 2,347,897 Febrey May 2, 19442,373,861 Van Inwagen Apr. 17, 1945 2,427,517 Wilson et al. Sept. 16,1947 2,513,147 Chorpening June 27, 1950 2,520,981 Thomson Sept. 5, 1951FOREIGN PATENTS 359,346 Great Britain Oct. 22, 1931 709,210 France Aug.4, 1931

